[Bug pch/49579] Document how to use PCH with Automake

[pinskia at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49579

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> 2011-06-29 
14:39:28 UTC ---
I think this should be an automake bug rather than a GCC one.